[llvm-commits] [llvm] r60275 - in /llvm/trunk: lib/Transforms/Scalar/InstructionCombining.cpp test/Transforms/InstCombine/apint-sub.ll test/Transforms/InstCombine/sdiv-1.ll test/Transforms/InstCombine/sub.ll

[Duncan Sands]

Hi Bill,

> I think that negate only overflows on minint, instead of:

if X has any bit zero, then it can't be minint.  So maybe
this transform can be done for non-constants by querying some
"what bits are set" mechanism.

> > +    // -X/C -> X/-C, if and only if negation doesn't overflow.

Also, I'm a bit confused about the tests.  As far as I can see,
what you care about is whether X overflows, not whether C overflows,
but you seem to test the later.  Clearly, if (-X) overflows then
the transform is wrong.  Now suppose (-C) overflows.  Then C is
minint, so -C = minint also.  Thus (-X)/C and X/(-C) are both
zero (i.e. the transform is correct) unless -X or X is minint,
which is the same as X = minint.  In otherwords, again the transform
is correct unless -X overflows.  Thus checking whether -X overflows
catches all cases when the transform is incorrect.

Ciao,

Duncan.